Myelin sheath allows fast saltatory conduction in PNS and CNS axons. This sheath is altered in diseases such as multiple sclerosis (CNS), Guillain-Barre and other neuropathies (PNS). Understanding how myelin is formed or repaired is crucial to the patient and requires basic studies of factors triggering differentiation of myelin-forming cells and their normal interaction with axons in vitro and in vivo. In the PNS, each Schwann cell makes a basement membrane before myelinating an axon. Although cultured rat Schwann cells do not synthesixe myelin components in the absence of axons, they synthesize essential components of their basement membrane: laminin and collagen type IV. Laminin, a large glycoprotein, is secreted into the medium and binds to Schwann cell membrane, as seen with TEM. In addition, purified laminin promotes adhesion and elongation of Schwann cells and neurite outgrowth of sensory ganglia in a dose-dependent and specific manner. Thus laminin might play a role in Schwann cell-neuron interaction in development and regeneration at a stage preceding myelination. In contrast to Schwann cells, isolated oligodendrocytes synthesize in vitro myelin components such as galactocerebroside and basic protein (BP) while developing a phenotype resembling their in vivo counterpart. We are studying how and when BP is first synthesized by these cells and plan to use sensitive techniques of in situ hybridization to also detect BP transcripts. We will also investigate if the myelin-associated glycoprotein (MAG) is expressed in the absence of neurons. MAG is an integral membrane glycoprotein which is consistently associated with the periaxonal space, maintains the axon-Schwann cell contact (12-14 nm wide) and is not found in compact myelin (as seen by immunocytochemical techniques) in normal and pathological nerves in vivo. MAG has also been shown to be an antigen recognized by monoclonal IGM in paraproteinemia patients using electroblots. The antigenic site recognized appears to be the carbohydrate moieties of the molecule.